Murine 3T3-L1 cells were used to study synthesis and secretion of lipoprotein lipase in adipocytes. Lipase protein was present in confluent, but not preconfluent, 3T#-L1 fibroblasts. The amount of enzyme in the cells increased many fold as cells differentiated into adipocytes. This increase was accompanied by parallel increases in lipase activity and secretion. The amount of (S-35) methionine incorporated into lipase by 3T3-L1 adipocytes increased for 2h and then remained constant, indicating a half-life of 1 h for newly synthesized lipase. Lipoprotein lipase released from cells by heparin was maximally active and accounted for 40% of active lipase associated with the cells. Tunicamycin reduced total protein synthesis 25%, increased cellular lipase protein 75%, decreased cellular lipase activity 99%, and blocked completely release of lipase to the medium. Molecular weight of (S-35)-labeled lipase synthesized in the presence of tunicamycin was smaller on SDS-PAGE than that synthesized by control cells, 48,000 vs 55,000, due to arrest of n-glycosylation of protein in endoplasmic reticulum. The complement of carbohydrates needed for full activity and secretion of lipoprotein lipase requires study. A preliminary finding that hepatic lipase activity was very low in liver and plasma of Combined Lipase Deficient (cld/cld) mice was confirmed by immunoinhibition studies. Lipoprotein lipase activity, which was very low in extra-hepatic tissues, was unexpectedly high, 40% of normal, in liver of cld/cld mice, suggesting that genetic control of this lipase in liver differs from that in other tissues.